1. Field of the Invention
The present invention relates to a projection scanning exposure apparatus which, at the projection of a pattern of a mask onto a photosensitive substrate, simultaneously scans said mask and said photosensitive substrate with respect to a projection optical system.
2. Related Background Art
In the conventional scanning exposure apparatus there is known a system employing a 1:1 reflective projection optical system and combining a mask stage supporting a mask and substrate stage supporting a photosensitive substrate (wafer) to a common movable column, thereby scanning said mask and substrate at a same speed. Said 1:1 reflective projection optical system, featured by limited chromatic aberration over a wide wavelength region due to the absence of refractive elements (lenses), enables high-speed scanning exposure by employing two or more spectral lines (for example g- and i-lines) from the light source (mercury lamp) for increasing the intensity of the light. In such reflective projection optical system, since the astigmatisms in the sagittal (S) image plane and in the meridional (M) image plane can be made zero only in a limited range around a certain image height from the optical axis of said optical, the exposing light illuminating the mask assumes the form of a part of an annular area, or so-called arc-shaped slit.
In such 1:1 scanning exposure apparatus (mirror projection aligner), if the image of the mask pattern projected onto the wafer does not constitute the mirror image of said mask pattern, the exposure operation can be completed by a one-dimensional scanning motion of a movable column to which the mask and the wafer are connected in a mutually aligned manner.
In a 1:1 projection optical system in which the image of the mask pattern projected on the wafer constitutes the mirror image of said mask pattern, the mask stage and the wafer stage have naturally to be moved in mutually opposed directions with a same speed.
Also in the conventional exposure systems, it is already known to incorporate a refractive element for increasing or decreasing the magnification of the projected image and to scan the mask stage and the wafer stage with a speed ratio corresponding to said magnification.
In such case, the projection optical system consists of a reflective element and refractive elements, or of refractive elements only. An example of a reduction projection system consisting of a reflective element and refractive elements is disclosed in U.S. Pat. No. 4,747,678, and a scanning exposure apparatus utilizing said projection optical system is commercialized as a step-and-scan aligner by Perkin and Elmer Co. under a commercial name Micrascan.
Furthermore, a method of step-and-scan exposure with a reduction projection optical system capable of full-field projection is disclosed in U.S. Pat. No. 4,924,257.
In such scanning exposure apparatus with non-unity projected magnification, it is necessary to precisely move the mask stage and the wafer stage in scanning motion with a speed ratio corresponding to the magnification, and to maintain the aberration of the mask pattern and the pattern on the wafer in the course of scanning exposure within a predetermined tolerance. The tolerable aberration, approximately defined by the minimum line width on the wafer, is only about 1/5 to 1/10 in case of forming, for example, a pattern with a line width of about 0.8 .mu.m.
Consequently, it is desirable that the positional aberration between the mask and the wafer can be constantly monitored during the scanning exposure.
As a conventional example, U.S. Pat. No. 4,697,087 discloses a method of correcting the mutual positional relationship of the reticle and the wafer (including magnification error and rotational error) by detecting plural broken chevron-shaped reticle marks formed on the reticle and plural broken chevron-shaped wafer targets formed on the wafer, in succession immediately before and during the scanning exposure.
The above-mentioned method is schematically illustrated in FIG. 1. Plural broken chevron-shaped marks RM1, RM2, RM3 formed in the peripheral area of a reticle R and plural broken chevron-shaped marks WM1, WM2, WM3 formed outside a shot area SA on a wafer W are mutually scanned by slit illuminating light beams AIL in broken chevron-shaped arrangement. In FIG. 1, for the purpose of simplicity, the magnification of the projection optical system is assumed as unity, and the reticle R and the wafer W are moved in mutually opposed directions as indicated by arrows. The amount of illuminating light beams AIL normally reflected or scattered by the reticle marks RM1, RM2, RM3 and the wafer marks WM1, WM2, WM3 is defined as a function of time as shown in FIGS. 2A and 2B.
FIG. 2A indicates an example of signal obtained by photoelectric detection of the light reflected from the reticle marks, and FIG. 2B indicates an example of signal obtained by photoelectric detection of the light scattered from the wafer marks. The alignment of the reticle and the wafer can be attained by regulating the scanning speeds and relative position of the reticle R and the wafer W in such a manner that a pulse P1 in the reticle signal matches in time paired pulses P2, P3 in the wafer signal, then a pulse P4 matches paired pulses P5, P6 and a pulse P7 matches paired pulses P8, P9. However, if these marks are detected for the first time in the course of scanning exposure, the precise alignment cannot be attained at the start of the scanning exposure. Therefore, a preliminary scanning is conducted as shown in FIG. 1 prior to the actual exposure, and the alignment error .DELTA.X in the scanning exposure direction is determined from the difference between the average position of the pulses P1, P4, P7 in the reticle signal and the average position of the pulses P2, P3, P5, P6, P8, P9 in the wafer signal. Also the alignment error .DELTA.Y in the orthogonal direction can be determined from the positions of the pulses P1-P6 as follows: EQU .DELTA.Y=((P5+P6)-(P2+P3))-(P4-P1).
In the conventional method explained above, since the signal wave is obtained only when a very fine slit-shaped light beam AIL crosses each alignment mark, it is indispensable, for attaining a high alignment precision, to detect the positions of plural marks and to calculate the average position thereof. Consequently a preliminary scanning is inevitable before the main scanning for exposure, and the throughput is therefore scarificed. Also in the conventional mark arrangement, even if the precision of detection of each mark is sufficient, the marks only have a very small width in the scanning direction. Consequently the pulses are inevitably intermittent in the alignment signal, and the alignment error is practically not determined during the interval of the pulses.
For this reason, in the interval between the plural marks arranged in the scanning direction, there has to be relied on the data obtained by laser interferometers for measuring the positions of the scanning stages for moving the reticle R and the wafer W.